The first is that the niche you mentioned is probably the only time it may be beneficial--but the schools that offer it do not place in the big firms that work in that area.

Second, you don't really need it at all: a Canadian JD is enough to take the bar in a few US states (MA and NY being the big ones, and some top firms take a few students every year from Canadian schools). Going the other way it is not that difficult to transition any US degree into Canadian eligibility: you just take a few basic exams (NCA exams) which most do not find that difficult.

Third, the Canadian legal market is notoriously skeptical of foreign degrees (there are quite a few schools that target Canadian students around the world) outside of Oxbridge. This is especially a problem since you have no alumni network, and much more importantly, will have an excruciating time finding an articling position.

Fourth, you are paying US tuition numbers (and in USD which is significant right now) for a Canadian degree where the vast majority of your peers have less than 1/3 your debt--starting salaries for lawyers reflects this difference.

Fifth, even the few jobs that kind of can make use of the dual degree will have you practicing in just one jurisdiction likely.

Figure out which country you want to work in and go there. If you are ambivalent than pick a choice that would be ok as if you were planning on practicing the country it is located in.